1. Field of the Invention
This invention relates to a liquid crystal display. In particular, the present invention is a method for fabricating a thin film transistor for a liquid crystal display, which can prevent blackening of a pixel electrode.
2. Description of the Related Art
A conventional method for fabricating a thin film transistor for a liquid crystal display is to be explained hereinafter, referring to FIGS. 1a-1h. First, as shown in FIG. 1a, a gate electrode 2 is formed by depositing and patterning metal on a transparent glass substrate 1, and as shown in FIG. 1b, a gate insulation film 3 is formed over the substrate 1 and the gate electrode 2. A semiconductor layer 4 is then formed over gate insulation film 3 and an n+ type ohmic contact layer 5 is formed over the semiconductor layer 4.
Next, as shown in FIG. 1c, the semiconductor layer 4 and the n+ type ohmic contact layer 5 are patterned to leave them only over an area on the gate insulation film 3 over the gate electrode 2. As shown in FIG. 1d, a pixel electrode 6 is then formed on a part of the gate insulation film 3 offset from the gate electrode 2, by depositing and patterning an ITO (Indium Tin Oxide) film.
Next, as shown in FIG. 1e, a metal barrier layer 7, a source electrode 8-1 and a drain electrode 8-2 are formed over the semiconductor layer 4 and the n+ type ohmic contact layer 5, except over the gate electrode 2. Source electrode 8-1 and drain electrode 8-2 are formed by forming a metal barrier layer and a metal layer for the source/drain electrodes, successively over the whole surface of the substrate, and patterning the metal barrier layer and the metal layer.
Because of the formation of the source/drain electrodes 8-1 and 8-2, an upper surface of the n+ type ohmic layer 5 over the gate electrode 2 is exposed. The source electrode 8-1 and the metal barrier layer 7 under the source electrode 8-1 are formed over the semiconductor layer 4 and the pixel electrode 6. The source electrode 8-1 and the metal barrier layer 7 under the source electrode 8-1 are therefore electrically connected to the pixel electrode 6.
As shown in FIG. 1f, the n+ type ohmic layer 5 exposed between the source/drain electrode 8-1 and 8-2 is then etched away. The patterned source/drain electrodes 8-1 and 8-2 and the metal barrier layer 7 are used as masks during the etching process. As a result of the etching, a surface of the semiconductor layer 4 under the ohmic layer is exposed.
As shown in FIG. 1g, a protective insulation layer 9 is deposited over the whole surface of the substrate. As shown in FIG. 1h, the protective insulation film 9 is then patterned to expose the pixel electrode 6. The thin film transistor for a liquid crystal display is thereby completed.
The foregoing conventional method for fabricating an LCD 10 TFT causes blackening. Transparent films of In.sub.2 O.sub.3 and SnO.sub.2 precipitate into non-transparent metals of In and Sn due to depletion of oxygen from the ITO film comprising the pixel electrodes because of H.sub.2 plasma during deposition of the protective insulation layer 9. When, as shown in FIG. 1g, the protective insulation layer 9 is deposited over the surface of the substrate, the pixel electrode 6 formed of ITO film is exposed, reducing rate of transmission of the pixel electrode and subsequently degrading the image quality.